Showing papers on "Helicopter rotor published in 2007"

Real-Time Nonlinear Embedded Control for an Autonomous Quadrotor Helicopter

Abstract: Control system design of aerospace vehicles with actuator saturation is an important practical design problem that many previous approaches to nonlinear autopilot design did not consider. In particular, small unmanned aerial vehicle rotorcraft actuators often have physical limitations such as a restricted onboard power supply. Disregard of actuator saturation can affect the final performance, but the reduction in performance can be mitigated if actuator saturation is included in the controller design. In this paper, we propose a nested-saturation-based nonlinear controller for the stabilization of a rotary-wing aircraft. This control strategy allows the incorporation of actuator magnitude saturation and has satisfactory dynamic performance. The nested-saturation technique enables the controller to ensure the global asymptotic stability of a quadrotor helicopter while improving the performance of the closed-loop system. By using Lyapunov analysis, the convergence property is established for the complete nonlinear model of the quadrotor rotorcraft. Simulation results show the performance of the proposed control strategy. Using embedded sensors and onboard control, we performed a real-time autonomous flight. Indeed, experimental results have shown that the proposed control strategy is able to autonomously perform the tasks of taking off, hovering, and landing.

Novel Boundary Integral Formulation for Blade-Vortex Interaction Aerodynamics of Helicopter Rotors

Abstract: A direct panel method based on a novel boundary integral formulation for the velocity potential is presented and applied to helicopter rotors experiencing blade-vortex interaction. It avoids the numerical instabilities arising in the standard direct panel method in case of blade/wake impingement. This aerodynamic formulation yields a unified approach for the calculation of free-wake evolution and the blade-pressure field; it is fully 3-D, includes body-thickness effects, and can be applied to blades with arbitrary shape and motion. Blade-pressure predictions and the corresponding acoustic fields correlate well with wind-tunnel test data for helicopter rotors in descent flight, in which severe blade-vortex interaction occurs.

A rigorous, engineer-friendly approach for modelling realistic, composite rotor blades

Abstract: A rigorous approach is presented for the modelling of composite beam structures of the type encountered in wind turbine blades, helicopter rotor blades and the like. The analysis methodology is based on a dimensional reduction of the geometrically nonlinear, threedimensional, anisotropic theory of elasticity. Small parameters stemming from the beamlike geometry of slender structures such as wind turbine blades lead to a splitting of the problem into a (usually) linear, two-dimensional cross-sectional analysis and a geometrically exact nonlinear, one-dimensional beam analysis. The incorporation of such beam analyses into flexible multibody codes presents a unified and powerful approach to the modelling of wind turbines with composite rotor blades. The generality and power of the methodology are illustrated in several examples. Copyright © 2006 John Wiley & Sons, Ltd.

Effects of a crack on the stability of a non-linear rotor system

Abstract: The stability of a rotor system presenting a transverse breathing crack is studied by considering the effects of crack depth, crack location and the shaft's rotational speed. The harmonic balance method, in combination with a path-following continuation procedure, is used to calculate the periodic response of a non-linear model of a cracked rotor system. The stability of the rotor's periodic movements is studied in the frequency domain by introducing the effects of a perturbation on the periodic solution for the cracked rotor system. It is shown that the areas of instability increase considerably when the crack deepens, and that the crack's position and depth are the main factors affecting not only the non-linear behaviour of the rotor system but also the different zones of dynamic instability in the periodic solution for the cracked rotor. The effects of some other system parameters (including the disk position and the stiffness of the supports) on the dynamic stability of the non-linear periodic response of the cracked rotor system are also investigated.

Recent developments in background oriented Schlieren methods for rotor blade tip vortex measurements

Abstract: The compressible blade tip vortex of rotary wings has been the subject of numerous investigations and its importance for the understanding of the helicopter flow field has been clearly emphasised. Due to its great impact on the dynamics of the flow field, the investigation of the tip vortex is directly linked to issues of flow control and aeroacoustic optimisation. However, among velocity field data, additional core density information on the blade tip vortex is desirable with a view to vortex modelling. In this work we describe an airborne background oriented Schlieren system for full-scale helicopter flight tests as well as the first results of the tomographic reconstruction of the compressible vortex core. We report the measurements of both a 0.4 Mach-scaled rotor model of the MBB BO 105 and the corresponding full-scale helicopter in hover flight condition. The tomographic reconstruction of the data allows us to estimate the density and the radius for the viscous core.

Helicopter rotor health monitoring- a review

Abstract: A review of the literature on helicopter rotor system health monitoring is conducted in this paper. An introduction is provided to the work on rotor track and balance and commercial health and usage monitoring systems. Research on the modelling of typical rotor system faults using aeroelastic analysis is discussed and the use of damage detection algorithms based on neural network, fuzzy logic, and system identification is pointed out. The use of non-destructive testing (NDT) approaches, such as modal methods, acoustic emission, and wave-based approaches for rotor health monitoring is discussed. Finally, work on the health monitoring of composite helicopter rotors is discussed and inverse problem solution and life prediction issues are addressed. Future research needs in the area are pointed out.

Aerodynamic Performance Analysis of a Gurney Flap for Rotorcraft Application

Abstract: In the present study, the aerodynamic characteristics of the Gurney flap were comprehensively investigated in terms of the performance requirements for a helicopter rotor by using two-dimensional Navier-Stokes equations. To this end, with the rotor operating flow conditions in mind, the static aerodynamic characteristics of the Gurney flap are thoroughly compared with those of the clean airfoil at various Mach numbers, incidences, and Gurney flap heights. Next, to understand the general dynamic stall features of the airfoils with the Gurney flap, a series of parametric studies are performed with respect to oscillating frequency and amplitude, and the correlations of the aerodynamic coefficients are obtained by using the dynamic stall function of Bousman. It is concluded that there exists some optimum Gurney flap heights, which minimize the drag and pitching moment while maximizing the lift coefficient and lift-to-drag ratio at the same time. In the present study, the 2% Gurney flap would be a good compromise to satisfy all the major rotorcraft design criteria.

Dual higher harmonic control (hhc) for a counter-rotating, coaxial rotor system

Abstract: A dual, counter-rotating, coaxial rotor system provides individual control of an upper rotor system and a lower rotor system. The lower rotor control system and the upper rotor control system provide six controls or “knobs” to minimize or theoretically eliminate airframe vibration. In a dual, counter-rotating, coaxial rotor system, application of a HHC system to the two rotor systems individually but located on the common axis, will yield essentially complete vibration reduction because the 6 controls will suppress the 6 loads.

Vibration Suppression of Nonlinear Rotor Systems Using a Dynamic Damper

Abstract: Due to the inevitable imbalance of rotating machinery, resonance occurs when the rotational speed is in the vicinity of the critical speed. A rotor system supported by a single-row deep groove ball bearing shows nonlinear spring characteristics due to the clearance of bearings. In this article, passive vibration control of nonlinear rotor systems using a dynamic damper is studied. Theoretical analysis is performed to investigate the influence of nonlinearity on the vibration characteristics of controlled rotor systems, and the theoretical results obtained are confirmed by experiments. An example shows that the fixed-point theorem for optimization of the dynamic damper cannot be used when the rotor system has an isotropic symmetrical nonlinearity. The Newton-Raphson method is used to determine the optimal parameters of the dynamic damper for the nonlinear rotor, and passive vibration control utilizing the dynamic damper is achieved in the nonlinear rotor system.

High-Resolution Computational and Experimental Study of Rotary-Wing Tip Vortex Formation

Abstract: The formation and rollup of a tip vortex trailed from a hovering helicopter rotor blade is studied in detail using both computations and measurements. The compressible Reynolds-averaged Navier-Stokes equations are computationally solved on an overset mesh system. The flow measurements are made using stereoscopic particle image velocimetry. The high resolution of both the numerics and the measurements reveal multiple coherent structures in the evolving rotor tip vortex flowfield. Secondary and tertiary vortices that result from crossflow separations near the blade tip are identified. These vortices, along with a part of the trailed wake, are ultimately entrained into the tip vortex that is formed downstream of the blade's trailing edge. The simulations clearly demonstrate the resolution required to accurately represent the complex three-dimensional flowfield. The advantage of particle image velocimetry, which has the ability to make planar measurements at a given instant of time, has been fully used to validate the computational fluid dynamics predictions. Even though linear eddy viscosity models are expected to inadequately represent the details of the turbulent quantities, good agreement is seen to be achieved with the particle image velocimetry measurements of the mean flowfield. The various sources of computational and measurement uncertainties are discussed.

Rotor Blade and Method of Making Same

Abstract: A helicopter rotor blade includes a structural composite skin defining a cavity therein; a composite spar disposed within the cavity and adhesively bonded to the skin, a portion of the spar exhibiting a C-shape in cross section; and a foam core disposed within the cavity and adhesively bonded to the skin. The structural composite skin forms an external, closed box structure configured to transmit mechanical loads encountered by the rotor blade.

Assessment of computational models for the effect of aeroelasticity on BVI noise prediction

Abstract: This paper deals with the computational analysis of acoustic fields generated by helicopter rotors when Blade-Vortex Interactions (BVI) occur. The prediction procedure starts from the determination of the steady periodic blade deformations. Then, the BVI-affected, unsteady aerodynamics solution is obtained by a potential-flow boundary integral formulation suited for aeronautical configurations experiencing blade-wake impingements. It is applicable to blades with arbitrary shape and motion and evaluates both wake distortion and blade pressure field. Finally, the noise field radiated by the rotor is computed through an aeroacoustic tool based on the Ffowcs Williams and Hawkings equation. The numerical investigation examines the sensitivity of BVI noise prediction on the aeroelastic model applied for the calculation of blade deformations, and assesses the accuracy of the results through correlation with experimental data concerning a helicopter main rotor in descent flight. Noise predicted is examined in ter...

Vibration-reducing and noise-reducing spoiler for helicopter rotors, aircraft wings, propellers, and turbine blades

Abstract: A spoiler attached to helicopter main rotor blades, tail rotor blades, propellers, aircraft wings, and machined into turbine blades, that reduces vibration and silences their operation. Also, when added to a substantial part of the trailing edges of its rotor blades, the spoiler eliminates the repetitive pop-pop sound common to current helicopter flight. Preferably, the spoiler is made from durable resilient materials that bend with resistance for high speed oscillation and it is secured on the top or bottom side, or both, of the trailing edge of a blade or wing. Further, the free edge of the spoiler exhibits a non-repeating pattern of feather-like projections that collectively break up vortex formation so that the next wing or blade traveling through the same location has clean air/fluid in which to move. In addition to noise reduction, the spoiler increases blade efficiency and wing lift.

Induced shear actuation of helicopter rotor blade for active twist control

Abstract: Induced shear based mechanism is used for attaining active twist in a soft-inplane hingeless rotor with a two-cell thin-walled airfoil section. The rotor properties dynamically represent a real rotor. A closed loop controller is developed to obtain the optimum voltage required to be given to the rotor blade for obtaining twist using strain rate feedback. Nonlinear relationship between piezoelectric shear coefficient and applied electric field is included in the controller design. Optimal placement of actuators lead to an overall vibration reduction of about 65%. Since thin-walled structures such as rotor blades are highly flexible and have strong aeroelastic effects, these effects on the loads and stability are thoroughly studied to evaluate the feasibility of the active twist rotor concept. No aeroelastic instabilities are found to occur due to active twist.

Aeromechanics Analysis of a Heavy Lift Slowed-Rotor Compound Helicopter

Abstract: A heavy lift slowed-rotor tandem compound helicopter was designed as a part of the NASA heavy lift rotorcraft systems investigation. The vehicle is required to carry 120 passengers over a range of 1200 nautical miles and cruise at 350 knots at an altitude of 30,000 feet. The basic size of the helicopter was determined by the United States Army Aeroflightdynamics Directorate's design code RotorCraft. Then performance, loads, and stability analyses were conducted with the Comprehensive Analytical Model of Rotorcraft Aerodynamics and Design II. Blade structural design (blade inertial and structural properties) was carried out using the loading condition from the Comprehensive Analytical Model of Rotorcraft Aerodynamics and Design II. A rotor parametric study was conducted to investigate the effects of the twist, collective, tip speed, and taper on aircraft performance. Designs were also developed for alternate missions to explore the influence of the design condition on performance.

Upper rotor control system for a counter-rotating rotor system

Abstract: An upper rotor control system is contained within an upper rotor shaft and upper hub assembly of a contra-rotating rigid rotor system. A collective servo assembly includes a hydraulic actuator that provides collective pitch to all blades through axial extension/retraction of the control rod relative the upper rotor shaft for collective pitch control of the rotor blades. The collective servo assembly includes a spherical bearing for attachment of the control rod to aircraft structure. An X-Y positioner assembly includes a bearing arrangement which allows the shaft to rotate, while the X-Y positioner assembly remains non-rotational therein. The X-Y positioner assembly includes a multitude of hydraulic actuators, orthogonally positioned, to tilt the control rod about the spherical bearing off the axis of rotation of the upper rotor shaft for cyclic pitch control of the rotor blades.

Fuzzy-Logic-Based Health Monitoring and Residual-Life Prediction for Composite Helicopter Rotor

Abstract: A health-monitoring and life-estimation strategy for composite rotor blades is developed in this work. The cross-sectional stiffness reduction obtained by physics-based models is expressed as a function of the life of the structure using a recent phenomenological damage model. This stiffness reduction is further used to study the behavior of measurable system parameters such as blade deflections, loads, and strains of a composite rotor blade in static analysis and forward flight. The simulated measurements are obtained using an aeroelastic analysis of the composite rotor blade based on the finite element in space and time with physics-based damage modes that are then linked to the life consumption of the blade. The model-based measurements are contaminated with noise to simulate real data. Genetic fuzzy systems are developed for global online prediction of physical damage and life consumption using displacement- and force-based measurement deviations between damaged and undamaged conditions. Furthermore, local online prediction of physical damage and life consumption is done using strains measured along the blade length. It is observed that the life consumption in the matrix-cracking zone is about 12-15% and life consumption in debonding/delamination zone is about 45-55% of the total life of the blade. It is also observed that the success rate of the genetic fuzzy systems depends upon the number of measurements, type of measurements and training, and the testing noise level. The genetic fuzzy systems work quite well with noisy data and are recommended for online structural health monitoring of composite helicopter rotor blades.

Rotorcraft simulation modelling and validation for control law design

Abstract: This paper describes the development and validation of a high fidelity simulation model of the Bell 412 helicopter for handling qualities and flight control investigations. The base-line model features a rigid, articulated blade-element formulation of the main rotor, with flap and lag degrees of freedom. The Bell 412 HP engine/governor dynamics are represented by a second-order system. Other key features of the base-line model include a finite-state dynamic inflow model and lag damper dynamics. The base-line model gives excellent agreement with flight-test data over the speed range 15-120kt for on-axis responses. Prediction of off-axis responses is less accurate. Several model enhancement options were introduced to obtain an improved off-axis response. It is shown that the pitch/roll off-axis responses in transient manoeuvres can be improved significantly by including wake geometry distortion effects in the Peters-He finite-state dynamic inflow model.

Modelling of a twin rotor system: A particle swarm optimization approach

Abstract: A scaled and simplified version of a practical helicopter, namely the twin rotor multi-input multi-output system (TRMS) is often used as a laboratory platform for control experiments. This system resembles a helicopter in many aspects. Since the TRMS permits both 1 and 2 degrees of freedom (DOF) motions, it can be considered as a static test rig for an air vehicle. This study presents investigations into modelling of the TRMS using particle swarm optimization (PSO). First, 1-DOF models are extracted for both vertical (pitch) and horizontal (yaw) channels independently. Then, a 2-DOF parametric model is developed taking cross-coupling between the channels into consideration. A particle swarm algorithm that uses time-varying inertia weight factor and time-varying acceleration coefficients is considered in this work. The effectiveness of the algorithm in modelling the system is validated and verified in terms of tracking, stability, and ability of derived model in capturing the dynamics of the system...

Chaotic Response of an Airfoil due to Aeroelastic Coupling and Dynamic Stall

Abstract: Flight-test data of helicopters indicate that vibratory levels in the fuselage exhibit a wide spectrum of frequencies, including a few below the rotor revolutions per minute. It is well known that helicopter blades operate in a complex aerodynamic environment, involving time-varying heave, pitch, and pulsating oncoming flow. During operation, some sections of the rotor blade undergo dynamic stall once in a revolution. This paper attempts to understand the reason for the existence of several frequencies in the response of the fuselage and the possible cause for this observed phenomenon by analyzing the effects of dynamic stall and aeroelastic couplings on the response of 2-D airfoil. The ONERA dynamic stall model developed by Petot is modified by incorporating a higher-order rational approximation of Theodorsen's lift deficiency function. This improved model is shown to provide a better correlation with experimental stall data. The response characteristics of a 2-D airfoil undergoing pitching and plunging motion in a pulsating oncoming flow, simulating the response of a cross section of a helicopter rotor blade in forward flight are analyzed. This study shows significant difference in the response characteristics of the airfoil for unsteady (dynamic stall model) and quasi-steady aerodynamic models. It is observed that the nonlinear aerodynamics (dynamic stall effects) in association with aeroelastic couplings above a certain level lead to a bounded chaotic motion of the airfoil.

Identification of equivalent modal damping for a wind turbine at standstill using Fourier and wavelet analysis

Abstract: The current paper presents approaches to evaluate the equivalent modal viscous damping ratios for a wind turbine tower, using Fourier and wavelet analysis based linear regression. In the Fourier analysis, the estimated damping ratios are constant, but in the wavelet analysis, temporally varying damping ratios using a time-segmented least squares approach can be identified. The estimation of time varying damping is important in assessing the risk of negative aeroelastic damping in wind turbines. In absence of experimental data, the proposed approaches have been illustrated on numerically simulated response obtained from a simple wind turbine model. The model used for generating the response time history data for the wind turbine tower is composed of a flexible tower and rotor blade system, inter-connected using a substructuring technique, which facilitates the blade/tower coupling. A model order reduction technique is first used to model each of the two substructures (tower/nacelle and rotor system...